In bypass turbojets, the shaft of the low-pressure turbine is hollow and may contain a tube, commonly referred to as a “center vent” tube that serves to connect the bearings supporting the low-pressure rotor to the outside atmosphere so as to discharge a flow of oil-laden air towards the downstream end of the turbojet.
In general, this tube extends over the entire length of the rotor shaft and its ends are constrained to rotate together with the shaft.
Given its long length and its relatively thin wall, the tube is conventionally supported by one or more centering rings that include means for bearing against the inside wall of the shaft.
Nevertheless, in order to improve the dynamic behavior of the turbojet, the low-pressure compressor shaft generally has an inside diameter that varies, thus presenting shoulders or narrowings.
To enable the centering rings to be put into place inside the shaft at a distance from its ends, the centering rings must be capable of going past these shoulders or narrowings in the inside surface of the shaft.
For this purpose, known centering rings are generally mounted by force using systems of the type comprising screws and spreaders, which can lead to the rings being wrongly positioned and can damage the inside surface of the shaft.
In addition, mounting such rings requires special tooling that is relatively expensive.
Furthermore, known centering rings are too bulky to be suitable for use in turbojets of small size, typically jets having a fan of diameter less than forty inches, i.e. about one meter.
Document DE 10 210 954-A1 describes an engine device comprising a cylindrical cage that is stationary in rotation and that houses a disk that is secured to a rotary shaft. The cage is carried by spokes of shape-memory material, with electricity being passed through the spokes in succession so as to heat them and shorten them in succession, thereby causing the disk to orbit inside the cage and drive the shaft secured to the disk in rotation.